The modern sporting rifle (MSR), based on the AR-15 platform, is one example of a gas operated firearm. An MSR appears cosmetically similar to military rifles, such as the M-16, but function like other semi-automatic civilian sporting firearms, firing only one round with each pull of the trigger. Gas operated firearms are also used by law enforcement and military organizations. Examples of gas operated firearms include, but are not limited to, AR10, AK-47, AK-74, M14 M16, M16A2, M4, FN SCAR family, M110, MK11, and others. These gas operated rifles have been produced by numerous manufacturers. These weapons, typically shoot, but are not limited to, 5.45 mm, 5.56 mm, 6.8 mm, and 7.62 mm bullets which provide very high bullet velocities.
These gas operated type rifles utilize either a direct gas impingement system or a gas and push rod system for operating their ejection and loading mechanisms, in an automatic mode and a semi-automatic mode. The expanding gas from the cartridge propellant is tapped from a port in the barrel intermediate the chamber and the muzzle end of the barrel. In the direct gas impingement system, a conduit extends from the port to the upper receiver and into the region of the bolt carrier. In the gas and pushrod system, the gas impinges against the push rod which extends to the upper receiver and into the region of the bolt carrier. During the initial firing of the cartridge, the bolt insert is locked into the barrel extension, the gas forces the bolt carrier backward a short distance to unlock the bolt. As the bolt carrier moves toward the butt of the gun, a bolt cam pin, forces the bolt to rotate, by this time the bullet has left the barrel. The inertia of the bolt and bolt carrier continues the rearward motion causing the bolt to extract the fired empty cartridge. A spring absorbs the rearward motion of the bolt and bolt carrier forcing the bolt and bolt carrier forward to engage the next cartridge in the magazine and push same into the chamber ready for firing.
The gas pressures for operating the gas operated style weapons are significant and with the 5.56 mm cartridges the exit velocities, typically in excess of 2700 feet per second (fps), substantially exceeding the sound barrier (about 1,126 fps). Associated with these velocities are high bullet travel distances, in excess of 2 miles, and high noise levels, including from the bullet breaking the sound barrier and generating shock waves that cannot be effectively suppressed.
Modifications have been developed for these gas operated weapons to shoot low mass rounds at low velocities that utilize telescoping cartridges-practice ammunition. Typically the cartridges have very low mass, compared to lethal rounds, and may also have frangible projectiles with marking media. The modifications include a bolt and bolt carrier modification that allows the bolt to retract entirely by the propulsion of the expanding telescoping cartridge with no assist from the gas port, effectively changing the function of the weapon from a direct gas impingement system to a direct blowback system. The bolt does not lock into place rearward of the chamber. The energetics in these cartridges is low compared to a normal lethal round and the rounds are relatively expensive.
An need remains for a system that implements a cartridge that fires projectiles at subsonic muzzle velocities, to be used with a modern sporting rifle that has energy levels in a mid energy range that may be used for hunting small game or target practice, that is not supersonic, and that does not have the distance range or energy levels of conventional cartridges, but still allows the modern sporting rifles to reliably cycle.